The very high melting points and consequent high temperature strength of the so called refractory metals, including but not limited to columbium, molybdenum, tantalum, rhenium and tungsten, have made them logical candidates for applications in high temperature environments. However, the unacceptably poor oxidation resistance of all of these metals or alloys thereof has in the past, limited their use to applications only in non-oxidizing environments (inert gas or vacuum atmospheres for all, reducing atmospheres for some).
A substantial number of programs, during the period from 1954-1970, aimed at the development of oxidation resisting coatings for the refractory metals, yielded some positive results, the most notable of which are covered by U.S. Pat. No. 3,540,863 issued Nov. 17, 1970, to Seymour Priceman and Lawrence Sama. The fused silicide coating is formed by applying a dried slurry of powdered silicon alloy on the substrate, then heating to a temperature and for a time to melt the alloy and react it with the substrate to form refractory metal silicides and finally cooling to ambient temperature. The fused refractory metal silicide coatings covered by this patent have been widely accepted and have proven successful in real service applications over the intervening years. The principal real service applications have been for liners in the afterburner nozzles of gas turbine engines (e.g., the Pratt & Whitney Aircraft F-100 engine) and for thrust chambers, thrust chamber-nozzle assemblies, and nozzle extensions for liquid rocket motors. In these applications the design wall temperatures of the component may range from 2200.degree. F. to 3000.degree. F., which is beyond the capability of conventional metals.
Use of fused silicide coated refractory metals in gas turbine engines places burdens on the silicide coating in addition to oxidation degradation. In particular, gas turbine engines currently employ a wide variety of metals, including aluminum, titanium, steels and of course nickel, cobalt and iron based superalloys in various engine components upstream of the fused silicide coated refractory metal hot section component. In addition a very large variety of materials are also used as coatings for wear, corrosion and erosion resistance and as abradable seals upstream of the coated refractory metal hot section component. Therefore, there is potential for many of these other materials to accidentally or inadvertently come in contact with the coated refractory metal component during any reasonable period of operation. Since the refractory metal component may be operating at 2200.degree.-3000.degree. F. and since the principal constituent of the protective oxidation resistant coating is silicon, contact of any of the above materials or oxide scales thereof with the refractory silicide coating can result in serious damage to the coating due to chemical or metallurgical reaction therewith and damage to the refractory metal substrate as a result of loss of coating protectiveness. If the metals or coating constituents of the upstream components come in contact with a silicide coated refractory metal at a surface temperature greater than 2000.degree. F. for a sufficient time, the metal may alloy or react with the silicide coating and result in either eutectic formation and/or localized melting of the coating, or at the very least, localized degradation of the coating which may then fail prematurely.
Degradation from oxidation and chemical/metallurgical reactions of dissimilar materials with the silicide coating will increase as engine manufacturers attempt to increase the temperature of the hot gas flow in gas turbine engines to enhance engine thrust and/or engine efficiency.